This is the description of the input section of the Tekscope, and I have an intermittent problem where channel 1 signal is at some negative saturated level offscreen, possibly out of common mode range. Sometimes, but rarely, it will work normally when I turn it on, and the signal sits at zero as it should. Has there been any components identified that could be causing this problem? Since it's intermittent, it seems that something isn't being biased correctly, but when it works, it works 100 percent, with no middle ground. Wondering if it might be a fractured solder joint, or maybe a failure of something like this:
Component number A1AT1
Tektronix part number 165–0034–00
Serial no. effective B030100
Name & description MICROCKT,HYBRID:ATTENUATOR/PREAMPLIFIER
Mfr.code TK2601
Mfr. part number 165–0034–00
THS720A MOD NV Component-Level Information Package
Component-Level Theory of Operation
This section describes the electrical operation of the THS720A MOD NV. Refer
to the schematics in the Component-Level Diagrams section as necessary.
Logic Conventions
This manual refers to digital logic circuits with standard logic symbols and
terms. Unless otherwise stated, all logic functions are described using the
positive logic convention: the more positive of the two logic levels is the high
(1) state and the more negative level is the low (0) state. Signal states may also
be described as “true” meaning their active state or “false” meaning their
non-active state. The specific voltages that constitute a high or low state vary
among the electronic devices.
Active-low signals are indicated by an asterisk (*) following the signal name,
for example IRQ*. Signal names are considered to be either active-high,
active-low, or to have both active-high and active-low states.
Oscilloscope System
Each of the two oscilloscope input signals enters the main board and passes
through an attenuator and preamplifier. Then each signal passes through an
isolation interface before reaching the trigger circuitry and sampler/digitizer. The
digitized signals are written into system memory for transfer to the display
system.
The acquisition system amplifies the input signals, samples them, converts them
to digital signals, and controls the acquisition process under direction of the
processor system. The acquisition system includes the trigger, acquisition timing,
and acquisition control circuitry.
Attenuators. Circuitry in the attenuator selects the input coupling and
attenuation factor. The processor system controls the attenuators with a serial
interface as well as with DC control voltages.
The Main Board assembly contains two attenuator hybrids (AT1 and AT2). To
allow floating measurements, the attenuators are individually isolated from the
rest of the Main Board by an isolation interface. Power to the attenuators is
coupled through transformers. Opto-isolators couple control voltages and DC
control signals to each attenuator. Transformers couple the high-frequency signal
path from each attenuator to the main board. Opto-isolators couple the low-fre-
quency signal path from each attenuator to the main board.
Each attenuator hybrid contains two attenuators, an AC coupling capacitor, three
relays, three relay drivers, and a preamplifier. The AC/DC coupling relay couples
the output of the BNC to the other relays in the attenuator hybrid. For AC
signals, the AC/DC coupling relay inserts a coupling capacitor into the input
signal path. The second relay selects a X10 attenuation factor. The third relay
selects a X100 attenuation factor.
edit: Interestingly, I just had the scope powered up for a while, and the channel just recovered on its own without any action on my part- I don't use the scope regularly, and maybe some part of the circuit is getting discharged from disuse. I'll have to see if using it more regularly prevents this from occurring, or if the problem is just a Fluke-